The present invention relates to a method of molding a plastic lens, and particularly but not exclusively relates to a plastic lens molding method which is suitable for molding spectacle lenses.
Molded lenses made of plastics are commonly used for spectacle lenses in place of glass lenses. In molding a plastic lens a pair of molds are usually used. The molds are arranged to form a gap which corresponds to the thickness of the lens, and the gap is sealed at the periphery thereof by a gasket to define a cavity between the molds. A thermosetting resin is injected into the cavity, and is then set by heating. Then, the molded lens is taken out by separating the molds and the gasket.
Such a molding method necessitates many gaskets according to thickness and powers of lens to be molded, and hence considerably raises cost in small-amount and many-kinds production of spectacle lenses.
To overcome such a disadvantage, an attempt has been made, in which a cavity is formed between molds, using an adhesive tape instead of a gasket, and a plastic material is injected into the cavity (Japanese examined patent publication No. 64 (1988)-2483, of which disclosure is incorporated herein by reference). In this earlier attempt, a pair of molds are held by mold jigs; then the molds are positioned to form a gap between them to correspond to the thickness of the lens to be molded; an adhesive tape is applied around the positioned molds to seal the gap to thereby produce a mold assembly which defines a cavity by the molds and the adhesive tape; and finally the mold assembly is taken out of the mold jigs.
In this conventional method, the alignment of the molds and sizing of the gap between the molds depend on the mold jigs. Thus, for different diameters of lenses, corresponding mold jigs must prepared. Particularly in the case of spectacle lenses, there are many kinds of lenses, which require many kinds of mold jigs. Moreover, high accuracy is required not only for mold jigs but also for supporting mechanisms of the mold jigs and other mechanisms of keeping the molds parallel and sizing the gap of the molds. These requirements make it hard to fabricate the mold jigs and the associated mechanisms.